Dimethyl carbonate (DMC) is an important industrial chemical which is typically synthesized from either phosgenation of methanol, alcoholysis of cyclic alkylene carbonate, or oxidative carbonylation of alcohols.
Each of the aforementioned processes has either commercial or technical disadvantages which make it desirable in the industry to develop a new process which avoids such set backs. For example, the phosgenation is a complicated and expensive process, alcoholysis is operationally and economically unfavorable since the production of the dialkyl carbonate always entails the simultaneous production of a stoichiometric amount of glycol, and the oxidative carbonylation process requires expensive technology for handling carbon monoxide under pressure and the avoidance of flammable mixtures in the reactor.
European Patent Publication No. 0 061 672 (Bayer AG) discloses a process for the preparation of carbonate esters by reacting alcohol and urea in the presence of a catalyst such as dibutyltin oxide (Bu.sub.2 SnO). Although this process overcomes the commercial and technological disadvantages of the phosgenation, alcoholysis, or oxidative carbonylation processes discussed above, as well as excess alcohol which is both costly and a reaction inhibitor, it is only capable of yielding carbonates at a conversion rate approaching 90%.
It would be most desirable to develop a process which has conversion and selectivity approaching 100%. In order to accomplish such a high percent of conversion of urea to carbamate and carbamate to carbonate the present inventors have discovered that it is important that (1) the mole fraction of alcohol be kept very low to avoid costly separation problems in the recovery section, (2) the catalyst be introduced in large amounts to avoid the formation of an unwanted azeotrope since excess catalyst suppresses the volatility of the alcohol, and (3) the amount of carbonate in the reactor should be kept low such that the formation of N-alkyl by-products is substantially eliminated. To maintain a low level of alcohol during the synthesis of carbonate, the present inventors have developed a novel organotin catalyst, i.e., a dialkyl isocyanato alkoxy tin(IV) and its derivatives, which is capable of exchanging its alkoxy group with the NH.sub.2 group of the alkyl carbamate. The ability of the catalyst to exchange its alkoxy group for the NH.sub.2 group allows for the use of significantly lower levels of alcohol than conventional process such as that disclosed in European Patent Publication No. 0 061 672 which relies solely upon the excess alcohol for removal of the NH.sub.2 group from the carbamate.
Since the process according to the present invention utilizes significantly less alcohol than earlier known processes, it greatly increases the rate of carbonate removal from the reactor via distillation without the need to distill and recycle huge quantities of unreacted alcohols. Larger quantities of alcohol cause the rate of carbonate synthesis to slow since the alcohol rather than the carbonate is refluxed out of the reactor vessel, whereby a high concentration of carbonate remains in the reactor which causes the formation of undesirable N-alkyl by-products.
The present invention also provides many additional advantages which shall become apparent as described below.